Curved-edge optical mode converter for heat-assisted magnetic recording

ABSTRACT

An apparatus includes an input coupler configured to receive light excited by a light source. A near-field transducer (NFT) is positioned at a media-facing surface of a write head. A layered waveguide is positioned between the input coupler and the NFT and configured to receive the light output from the input coupler in a transverse electric (TE) mode and deliver the light to the NFT in a transverse magnetic (TM) mode. The layered waveguide comprises a first layer extending along a light-propagation direction. The first layer is configured to receive light from the input coupler. The first layer tapers from a first cross track width to a second cross track width where the second cross track width is narrower than the first cross track width. The layered waveguide includes a second layer that is disposed on the first layer. The second layer has a cross sectional area in a plane perpendicular to the light propagation direction that increases along the light propagation direction. The cross sectional area of the second layer is smaller proximate to the input coupler and larger proximate to the NFT. The layered waveguide includes an interface between the first layer and the second layer, the interface comprises a curve.

RELATED APPLICATIONS

This application claims the benefit of Provisional Patent Application Ser. No. 62/560,802 filed on Sep. 20, 2017, which is hereby incorporated herein by reference in its entirety.

SUMMARY

Embodiments described herein are directed to an apparatus comprising an input coupler configured to receive light excited by a light source. A near-field transducer (NFT) is disposed at a media-facing surface of a write head. A layered waveguide is disposed between the input coupler and the NFT and is configured to receive the light output from the input coupler in a transverse electric (TE) mode and deliver the light to the NFT in a transverse magnetic (TM) mode. The layered waveguide comprises a first layer extending along a light-propagation direction and configured to receive light from the input coupler, the first layer tapering from a first cross track width to a second cross track width, the second cross track width being narrower than the first cross track width. A second layer is disposed on the first layer, the second layer having a cross sectional area in a plane perpendicular to the light propagation direction that increases along the light propagation direction, the cross sectional area being smaller proximate to the input coupler and larger proximate to the NFT. The layered waveguide comprises an interface between the first layer and the second layer, the interface comprises a curve.

Embodiments are directed to an apparatus comprising an input coupler configured to receive light excited by a light source. A near-field transducer (NFT) is disposed at a media-facing surface of a write head. A layered waveguide is disposed between the input coupler and the NFT and is configured to receive the light output from the input coupler in a transverse electric (TE) mode and deliver the light to the NFT in a transverse magnetic (TM) mode. The layered waveguide comprises a first layer extending along a light-propagation direction and configured to receive light from the input coupler, the first layer tapering from a first cross track width to a second cross track width, the second cross track width being narrower than the first cross track width. A second layer is disposed on the first layer, the second layer having a cross sectional area in a plane perpendicular to the light propagation direction that increases along the light propagation direction, the cross sectional area being smaller proximate to the input coupler and larger proximate to the NFT. The layered waveguide comprises an interface between the first layer and the second layer, the interface comprises a cubic plane curve.

The above summary is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The figures and the detailed description below more particularly exemplify illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings, where like reference numerals designate like elements, and wherein:

FIGS. 1A and 1B are perspective views of a hard drive slider that includes a waveguide in accordance with embodiments described herein;

FIG. 2 is a cross-sectional view shows details of a HAMR apparatus according to various implementations;

FIGS. 3A and 3B show the mode conversion region of the curved layered waveguide according to various embodiments;

FIG. 4A illustrates a cross section of the waveguide system having a first layer, a second layer, and a stop layer in accordance with some aspects;

FIGS. 4B and 4C illustrate a cross-section of the mid-point of the mode conversion section according to various embodiments;

FIGS. 5A-5F illustrate a process for forming a layered waveguide in accordance with embodiments described herein;

FIGS. 6 and 7 are flow diagrams that describe a processes of creating the layered waveguide structure according to various implementations;

FIGS. 8A-8E illustrate cross sections of layered waveguides having different cladding configurations according to various embodiments described herein; and

FIG. 9 illustrates a graph of how the polarization changes as the light propagates through the device in the curved design and the straight-edge design in accordance with some implementations.

The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.

DETAILED DESCRIPTION

The present disclosure is generally directed to read-write heads used in magnetic recording devices such as hard drives. In particular, this disclosure relates to heat-assisted magnetic recording (HAMR), which can be used to increase areal data density of magnetic media. In a HAMR device, information bits are recorded in a storage layer at elevated temperatures in a specially configured magnetic media. The use of heat can overcome superparamagnetic effects that might otherwise limit the areal data density of the media. As such, HAMR devices may include magnetic write heads for delivering electromagnetic energy to heat a small confined media area (spot size) at the same time the magnetic write head applies a magnetic field to the media for recording.

A HAMR read/write element, sometimes referred to as a slider, recording head, read head, write head, read/write head, etc., includes magnetic read and write transducers similar to those on current hard drives. For example, data may be read by a magnetoresistive sensor that detects magnetic fluctuations of a magnetic media as it moves underneath the sensor. Data is written to the magnetic media by a write coil that is magnetically coupled to a write pole. The write pole changes magnetic orientation in regions of the media as it moves underneath the write pole in response to an energizing current applied to the write coil. A HAMR slider will also generally include a source of energy, such as a laser diode, to heat the media while it is being written to by the write pole. An optical delivery path is integrated into the HAMR slider to deliver the energy to the surface of the media.

The optical delivery path of a HAMR slider may include a plasmonic transducer proximate a media-facing surface (e.g., air-bearing surface, contact surface). The plasmonic transducer shapes and transmits the energy to a small region on the medium. The plasmonic transducer is sometimes referred to as a near-field transducer (NFT), optical antenna, surface plasmon resonator, etc., and may include a plasmonic metal such as gold, silver, copper, aluminum, etc., and alloys thereof. The plasmonic transducer for a HAMR device is very small (e.g., on the order of 0.1 to a few light wavelengths, or any value therebetween) and creates a localized region of high power density in the media through an electromagnetic interaction. This results in a high temperature rise in a small region on the media, with the region exceeding the Curie temperature having dimensions less than 100 nm.

A light source may have a TE-like (transverse electric) output beam profile. However, a transverse magnetic (TM) mode may be desired for coupling with some configurations of plasmonic transducers. Embodiments below include a waveguide system that provides mode order conversion rotation including a TE to TM mode. It will be understood that the embodiments below may be configured to excite other modes in some configurations.

According to various implementations, a waveguide is used to convert input TE mode light from a light source into TM mode light. An optical waveguide has a cross sectional area in a plane perpendicular to the light propagation direction that increases nonlinearly along the light propagation direction. This is accomplished by using a layered waveguide having two or more layers. The TE mode light is input into the waveguide in a first layer. As light travels along the light propagation direction a second waveguide layer at least partially overlaps the first waveguide layer in a mode conversion region. The mode conversion region causes a rotation of the polarization of the light as it propagates along the length of the waveguide. The converted light then exits the waveguide in a TM mode. The length, width and/or height of the mode converter region can be adjusted to give partial or total conversion from TE to TM.

Embodiments are directed to an optical mode rotator for converting TE polarized light to TM polarization. TM polarized light may be used to operate a heat-assisted magnetic recording (HAMR) head. In some cases the HAMR head utilizes a plasmonic gap waveguide (PGW). A PGW configuration includes a near field transducer (NFT) that is a part of the waveguide.

In reference to FIG. 1A, a perspective view shows a HAMR slider assembly 100 according to an example embodiment. The slider assembly 100 includes a laser diode 102 located on input surface 103 of a slider body 101. In this example, the input surface 103 is a top surface, which is located opposite to a media-facing surface 108 that is positioned over a surface of a recording media (not shown) during device operation. The media-facing surface 108 faces and is held proximate to the moving media surface while reading and writing to the media. The media-facing surface 108 may be configured as an air-bearing surface (ABS) that maintains separation from the media surface via a thin layer of air.

The laser diode 102 delivers light to a region proximate a HAMR read/write head 106, which is located near the media-facing surface 108. The energy is used to heat the recording media as it passes by the read/write head 106. Optical coupling components, such as a mode converting waveguide system 110, are formed integrally within the slider body 101 (near a trailing edge surface 104 in this example) and function as an optical path that delivers energy from the laser diode 102 to the recording media via a near-field transducer 112. The near-field transducer 112 is near the read/write head 106 and causes heating of the media during recording operations.

The laser diode 102 in this example may be configured as either an edge-emitting laser or surface-emitting laser. Generally, the edge-emitting laser emits light from near a corner edge of the laser and a surface emitting laser emits light in a direction perpendicular to a surface of the laser body, e.g., from a point near a center of the surface. An edge-emitting laser may be mounted on the top surface 103 of the slider body 101 (e.g., in a pocket or cavity) such that the light is emitted in a direction parallel to (or at least non-perpendicular to) the media-facing surface. A surface-emitting or edge-emitting laser in any of these examples may be directly coupled to the slider body 101, or via an intermediary component such as a submount (not shown). A submount can be used to orient an edge-emitting laser so that its output is directly downwards (negative y-direction in the figure).

In the present disclosure, hard drive recording heads may use a different type of laser than what is shown in FIG. 1A. A read/write head 120 using this alternate approach is shown in FIG. 1B, wherein components are given the same reference numbers as analogous components in FIG. 1A. At least part of a semiconductor laser 122 or material to form a laser (e.g., epitaxial layer) is not self-supporting (e.g., not a separately packaged device) but is physically transferred to a target read/write head substrate that does contain already or will contain, after further processing, the other components of the read/write head (e.g., write coil and poles, reader stack) without the use of a separate or intermediate support during attachment. Carrying the semiconductor laser 122 with the read/write head substrate, without a separate or intermediate support substrate, can help to reduce the size and simplify the shape and connection methods, and it can also allow for the use of laser geometries and designs that are very different from simple edge-emitting cleaved facet lasers that have been proposed in the past.

In at least some cases, parts of the laser 122 (e.g., GaAs active region) are incompatible with epitaxial growth on the target substrate of a slider, which may be formed of a dielectric such as alumina. As such, the laser 122 cannot be formed using the same layer deposition processes used to form the magnetic and optical components that are integrated into the head. In embodiments described below, the laser may instead be formed on the substrate by transfer printing a thin, non-self-supporting crystalline layer (epitaxial layer), or a stack of such layers, from a growth substrate on which they were formed to a target substrate. Thereafter, the epitaxial layer and substrate are further processed (e.g., masked etched, further layers added) to form the integrated laser diode unit 122. This process of transferring non-self-supporting layers of epitaxial-growth-incompatible layers is referred to herein as On-Wafer Laser (OWL) process integration. This process may also be referred to as transfer printing, dry transfer printing, nanoprinting, etc. Embodiments described herein may be implemented in an OWL system.

The waveguide system 110 discussed herein and shown in FIGS. 1A and 1B may be applicable to any type of light delivery configuration. For example, a laser may be mounted on the trailing edge surface 104 instead of the top surface 103. In another configuration known as free-space light delivery, a laser may be mounted external to the slider 100, and coupled to the slider by way of optic fiber and/or waveguide. An input surface of the slider body 101 may include a grating or other coupling feature to receive light from the laser via the optic fiber and/or waveguide.

In reference now to FIG. 2, a cross-sectional view shows details of a HAMR apparatus 200 according to an example embodiment. Near-field transducer 112 is located proximate a media-facing surface 202 (e.g., ABS), which is held near a magnetic recording media 204 during device operation. In the orientation of FIG. 2, the media-facing surface 202 is arranged parallel to the x-z plane. A waveguide core 206 may be disposed proximate the NFT 112, which is located at or near the media writing surface 214.

The waveguide core 206 surrounded by cladding layers 208, 210. The waveguide core 206 and cladding layers 208, 210 may be made from dielectric materials such as Al₂O₃, SiO_(x)N_(y), SiO₂, Ta₂O₅, TiO₂, ZnS, SiN_(x), Nb₂O₅, AlN, Hf₂O₃, Y₂O₃, AlO, etc. Generally, the dielectric materials are selected so that the refractive index of the waveguide core layer 206 is higher than refractive indices of the cladding layers 208, 210. This arrangement of materials facilitates efficient propagation of light through the waveguide. Light is delivered from the waveguide core 206 along the negative y-direction where it is coupled to the NFT 112. A portion of the waveguide core (not shown) converts the light from a TE mode to a TM mode. The NFT 112 delivers surface plasmon enhanced, near-field electromagnetic energy along the y-axis where it exits at the media writing surface 214. This may result in a highly localized hot spot (not shown) on the media surface 214 when the media 204 placed in close proximity to surface 202 of the apparatus 200. Further illustrated in FIG. 2 is a recording pole 212 of the read/write head that is located alongside the NFT 112. The recording pole 212 generates a magnetic field (e.g., perpendicular field) used in changing the magnetic orientation of the hotspot during writing.

According to various embodiments described herein, a layered waveguide may be used. The layered waveguide has a mode conversion region that converts light from an input TE mode into a TM mode. Light is received from the input coupler in a section of the waveguide having first cross sectional area in a plane perpendicular to the light propagation direction. The cross sectional area may increase as the light travels through the mode conversion region of the waveguide. This may be achieved by using two or more layers to create the different cross sectional area along the light propagation direction. Embodiments described herein relate to a layered waveguide having an interface between the first layer and the second layer that is nonlinear. A straight-edge mode converter is etched along a substantially straight diagonal line. The straight-edge mode converter design is described in more detail in commonly owned U.S. Pat. No. 9,792,937, filed Jul. 8, 2016, which is incorporated by reference in its entirety.

The nonlinear design takes advantage of the fact that the speed of polarization change is nonlinear as the light travels along the structure. This may cause the light to rotate faster at the start and end of the mode converter, while maintaining substantial mode stability in the center where the natural speed of rotation is fastest. While embodiments described herein show a system having nonlinear interface with a curved design, it is to be understood that the a nonlinear interface may include other types of nonlinear interfaces that cause the mode rotation to rotate faster at the start and end while maintaining mode stability in the center of the mode converter. In some cases, the nonlinear interface may include at least one step instead of or in addition to the curve.

In accordance with various embodiments described herein, utilizing a curved design may significantly reduce the length of the device compared to the straight-edge version. A shorter device length may be useful for some implementations. For example, an implementation using an OWL system has limited space available to bend the light from the laser to the air-bearing surface (ABS).

FIG. 3A shows a top view of a layered waveguide in accordance with embodiments described herein. A first waveguide layer 320 includes a first tapered region 325 at a TE mode input end, a straight portion having a constant cross-sectional width, and a second tapered region 345 where light is output in substantially a TM mode. FIG. 3A further illustrates the waveguide with a second waveguide layer 340 on the first waveguide layer 320. The second waveguide layer partially overlaps with the first waveguide layer 320 in a mode conversion region 330 and slowly tapers to completely overlap with the first waveguide layer 320 at or near the second taper of the first waveguide layer 320. The light then exits the waveguide in a TM mode.

According to various configurations, an interface between the first waveguide layer 320 and the second waveguide layer 340 is nonlinear. For example, in FIG. 3A the interface between the first waveguide layer 320 and the second waveguide layer 340 comprises a curve. In some cases, the interface between the first waveguide layer 320 and the second waveguide layer 340 comprises a cubic plane curve. The change in cross-sectional structure of the waveguide at the mode conversion region 330 causes a rotation of the polarization of the light as it propagates along the length of the waveguide.

The dimensions of the layered waveguide may vary depending on design considerations. For example, the dimensions may vary based on the materials of the waveguide and/or the cladding layers. The length of the mode conversion region may enable an optimal conversion of TE mode light into TM mode light. For example, the length may be based on achieving a peak amount of TM mode light to exit the mode conversion region or may be based on achieving at least a predetermined percentage of TM mode light exiting the waveguide. The length of the interface region, L_(i) may be in the range of about 15-25 μm, for example. In some cases, the mode conversion region is about 20 μm.

Similarly, the width of the mode conversion region and/or of the input and output ends of the waveguide may be based on an optimal or desired amount of conversion between the TE mode and TM mode. For example, the width of the mode conversion region may be in the range of about 0.15-0.65 μm or about 0.3-0.35 μm. In some cases, the width of the mode conversion region is about 0.327 μm. The width of the input end of the waveguide, Ww1, may have the same value as the width of the output end of the waveguide, Ww2. According to various configurations, Ww1 is in a range of about 0.45 μm to about 0.85 μm. In some cases, Ww1 is about 0.65 μm. According to various configurations, Ww2 is in a range of about 0.7 μm to about 1.1 μm. In some cases, Ww2 is about 0.92 μm.

The length of the first and second taper may also change based on the materials or the cladding configuration of the waveguide system. The length of the first waveguide taper, TL1, and the second waveguide taper, TL2, may have the same or different values. For example, TL1 and/or TL2 may be in a range of about 2-6 μm. In some cases, TL1 and/or TL2 is about 4 μm. L1 and L2 represent the respective lengths of first and second portions between the interface region and the tapered regions as shown in FIG. 3A. L1 and L2 may have the same or different values. For example, L1 and/or L2 may be in a range of about 2 μm to about 10 μm or in a range of about 4 μm to about 8 μm. In some cases, L1 and/or L2 is about 6 μm.

FIG. 3B illustrates a more detailed view of the mode conversion region having a curved mode converter in accordance with embodiments described herein. The points P₀, P₁, P₂, P₃ are defined in terms of s_length1, s_length2, θc1, and θc2. According to various embodiments, s_length1 and s_length2 may have the same or different values. For example, s_length 1 and/or s_length2 is in a range of about 5 μm to about 13 μm or in a range of about 7 μm to about 11 μm. In some cases, s_length1 and/or s_length2 is about 9 μm. Similarly θc1, and θc2 may be the same or different values. For example, θc1, and/or θc2 may be in a range of about 0.8 degrees to about 2.8 degrees. In some cases, θc1, and/or θc2 is about 1.8 degrees. In some cases, the extent of the etch interface between the layers is defined by the cubic Bezier equation as defined by Equation 1. B(t)=(1−t)³ P ₀+3(1−t)² tP ₁+3(1−t)t ² P ₂ +t ³ P ₃, where 0≤t≤1  Equation 1

FIG. 4A illustrates a cross section of the waveguide system having a first layer 420 and a second layer 440. In some embodiments, a stop layer 435 is disposed between the first waveguide layer 420 and the second waveguide layer 440. FIGS. 4B and 4C show a cross-section of the mid-point of the mode conversion section along the propagation direction in which the second waveguide layer 442 starts to overlap with the first waveguide layer 422.

The height of the various waveguide layers may depend on a desired and/or optimal conversion of TE mode light into TM mode light. The height of the first layer (h1) may be in the range of about 0.05-0.4 μm The height of the second layer (h2) may have substantially the same value or a different value than that of h1. According to various embodiments, h2 is in a range of about 0.05-0.4 μm. According to various configurations h1 and/or h2 is about 0.17 μm.

The total height of the waveguide may vary depending on the position of a cross section perpendicular to the light propagation direction. Moving along the light propagation direction, the second waveguide layer 340 partially overlaps the first waveguide layer 320 within the mode conversion region of the layered waveguide. The value the total height at the overlapping portion is the sum of h1, h2, and the height of any intermediate layers, e.g., stop layer for formation of the layered waveguide design. While FIGS. 4A-4C show the layered waveguide with a stop layer, it is to be understood that the layered waveguide structure may not include a stop layer.

The angle at which the second waveguide layer 442 interfaces with the first waveguide layer 422 and/or the stop layer 435 may vary based on design characteristics. For example, the second waveguide layer may interface the stop layer at about a 90° angle as shown by θ₁ 470 in FIG. 4B. In some cases, the interface angle is greater than 90° as shown by θ₂ 480 in FIG. 4C. For example, the interface angle may be about 90°-150° or about 90°-110°, for example.

FIGS. 5A-5F illustrate a process for forming a layered waveguide in accordance with embodiments described herein. A first waveguide layer 520 is deposited. A stop layer 535 is then formed on top of the first waveguide layer 520. The stop layer 535 may be formed of Yttria, for example. A second waveguide layer 540 is formed on the stop layer 535. The first waveguide layer 520 and the second waveguide layer 540 can be made of NbO_(x), for example. In some cases, the first and second waveguide layers are made of different materials.

In FIG. 5B, a first mask 545 is applied to an exposed surface of the second waveguide layer 540. The exposed region 545 is etched to the stop layer 535. After the etching, the exposed section 455 can be filled with a cladding material 557 as shown in FIG. 5C. The cladding material may be SiO₂ or AlO, for example. In some cases, more than one cladding material is added to the etched out region.

A second mask 560 is then used for a second etching and/or milling process as shown in FIG. 5D. The exposed sections 575, 565 are milled to create the final waveguide structure. The milled or etched out regions are then filled in with cladding material 558. The resulting structure is shown in FIG. 5E. According to various implementations, the second mask and resulting etching/milling process is applied before the first mask. A bottom view of the resulting waveguide is shown in FIG. 5F. While FIGS. 5A-5F show the process of creating a layered waveguide using a stop layer, it is to be understood that the formation of the layered waveguide structure may be done without the stop layer.

According to various implementations, the process for creating the layered waveguide allows for the first mask or the second mask to have positional variations. For example, the mask may be designed to allow for a variation of 50 nm or less and still have substantially the same percentage of output TM. These variations may be horizontal or vertical. This slight variation may result in different start and end positions of the mode conversion region. If the variation of the mask overlay is known, the effective lengths of the mode conversion region can be calculated using the variation value and the constant angle

FIG. 6 is a flow diagram that describes a process of creating the layered waveguide structure in accordance with various embodiments described herein. A first waveguide layer is deposited 710. Optionally, a stop layer is formed 720 on the first waveguide layer. A second waveguide layer is deposited 730 on the stop layer and/or the second waveguide layer. A first mask is applied 740 to a substrate parallel surface of the second waveguide layer. The mask creates a masked portion and an exposed portion of the substrate parallel surface of the second waveguide layer. In some cases, the exposed region of the second waveguide layer is etched 750 to the stop layer. The etched region may be filled with a cladding material. A second mask is applied 752 to the second waveguide layer. The exposed regions along a substrate parallel surface of the second waveguide layer, the first waveguide layer, and the stop layer are etched and/or milled 754. The etched region may be filled with a cladding material having a different index of refraction from the first waveguide layer and the second waveguide layer.

According to various implementations, after the first mask is applied to the second waveguide layer, the exposed regions along a substrate parallel surface of the second waveguide layer, the first waveguide layer, and the stop layer are etched and/or milled 755 and a cladding material is added to the etched portion. A second mask is applied 757 to the second waveguide layer. Exposed regions of the second waveguide layer are etched 759 and filled with a cladding material.

FIG. 7 shows a flow diagram that describes a process for creating the layered waveguide structure without using a stop layer. A waveguide layer is deposited 760. A first mask is applied to the waveguide layer 765. The mask creates a masked portion and an exposed portion of the substrate parallel surface of the waveguide layer. In some configurations, an exposed region of the waveguide layer is etched 770 to a predetermined thickness. In some cases, the predetermined thickness is about halfway through the thickness of the waveguide layer. A second mask is applied 772 to the waveguide layer. The exposed regions of the waveguide layer are etched to a total thickness of the waveguide layer. In some cases, after applying the first mask to the waveguide layer, exposed regions of the waveguide layer are etched 780 to a total thickness of the waveguide layer. A second mask is applied 782 to the waveguide layer. Exposed regions of the waveguide layer are etched 784 to a predetermined thickness. In some cases, the predetermined thickness is about halfway through the thickness of the waveguide layer. As in FIG. 6, the etched regions may be filled with a cladding material having a different index of refraction from the waveguide layer.

The processes of FIGS. 6 and 7 result in a structure where the waveguide layer has a cross sectional area in a plane perpendicular to the light propagation direction that increases along the light propagation direction, the cross sectional area being smaller proximate to an input coupler and larger proximate to an NFT.

The layered waveguide system may have various cladding configurations that surround the layered waveguide in accordance with various embodiments described herein. FIG. 8A includes a first waveguide layer 820, a stop layer 835, and a second waveguide layer 840. A cladding layer 850 is disposed around the layered waveguide. The cladding layer may comprise a dielectric material such as SiO₂, for example. As stated previously, the stop layer is optional and may not be included. For example, FIG. 8E illustrates a layered waveguide system without a stop layer.

The dimensions of the layered waveguide may be different depending on the materials of the cladding and the waveguide layers. The dimensions of the mode conversion section of the waveguide may be based on an amount of output TM from the mode conversion region. For example, the width of the mode conversion section of the waveguide may be in the range of about 270-350 nm or be in the range of about 330-350 nm. According to various implementations, the width of the mode conversion region is about 325 nm. The length of the mode conversion region of the waveguide may be about 17-37 μm or may be about 22-27 μm. According to various implementations, the length of the mode conversion region is about 24 μm. The length of the mode conversion region of a waveguide material of TaO_(x) may be about 10 μm higher than the length of the mode conversion region having a NbO_(x) waveguide material. The chosen length may correspond to a desired percentage of TM provided at the output of the mode conversion region of the waveguide. The desired amount of TM may correspond to a peak amount of TM or may be a different value.

FIG. 8B illustrates another layered waveguide system in accordance with various embodiments. Similarly to FIG. 8A, FIG. 8B includes a first waveguide layer 820, an optional stop layer 835, and a second waveguide layer 840. Side cladding 852 surrounds the exposed portions of the second waveguide layer 840 and the stop layer 835 and portions of the exposed portions of the first waveguide layer 820. A bottom cladding layer 853 may be in contact with the first waveguide layer 820. A top cladding layer 851 is also used as shown in FIG. 8B. According to various embodiments, the top cladding layer 851 and the bottom cladding layer 853 comprise the same or similar materials. For example, the top cladding layer 851 and the bottom cladding layer 853 may comprise SiO₂. According to various implementations, the top cladding layer 851 and the bottom cladding layer 853 comprise different materials. The side cladding layer 852 may comprise a different material than either of the top cladding layer 851 or the bottom cladding layer 853. For example, the side cladding layer 852 may comprise AlO. The configuration shown in FIG. 8B may have a longer mode conversion region length than the configuration of FIG. 8A. For example, the mode conversion region may have a length between 80 and 125 μm, for example. The width of the mode conversion region of the waveguide may be between 200 and 250 nm for example. In some cases, the width is about 300 nm and the length is about 125 μm.

FIG. 8C shows another layered waveguide system in accordance with various embodiments. Similarly as in FIGS. 8A and 8B, FIG. 8C includes a first waveguide layer 820, an optional stop layer 835, and a second waveguide layer 840. FIG. 8C has a similar cladding configuration as that of FIG. 8B except that the side cladding layer 855 is thinner and the top cladding layer 854 extends along the sides of the side cladding layer 855. The thin side cladding 855 surrounds the exposed portions of the second waveguide layer 840 and the stop layer 835 and portions of the exposed sections of the first waveguide layer 820. A bottom cladding layer 856 may be in contact with the first waveguide layer 820. An extended top cladding layer 854 may also encompass portions of the side cladding layer 855. According to various embodiments, the top cladding layer 854 and the bottom cladding layer 856 comprise the same or similar materials. For example, the top cladding layer 854 and the bottom cladding layer 856 may comprise SiO₂. According to various implementations, the top cladding layer 854 and the bottom cladding layer 856 comprise different materials. The thin side cladding layer 855 may comprise a different material than either of the top cladding layer 854 and the bottom cladding layer 856. For example, the thin side cladding layer 855 may comprise AlO.

For the cladding configuration shown in FIG. 8C, the width and length of the mode conversion region of the waveguide may depend on the thickness (t) of the thin side cladding layer 855. A configuration with a thinner side cladding layer may result in a greater length and a greater width than a configuration with a thicker side cladding layer, for example. For a side cladding layer 855 thickness of about 100 nm, the width of the mode conversion region may be in the range of about 260-280 nm or about 270 nm. The length of the mode conversion region having using a thin side cladding layer 655 of about 100 nm may be in the range of about 17-28 μm or be about 20 μm, for example. For a thin side cladding layer 855 thickness of about 70 nm, the width of the mode conversion region may be in the range of about 260-290 nm or about 285 nm. The length of the mode conversion region using a side cladding layer of about 70 nm may be in the range of about 17-27 μm or be about 23 μm, for example. For a side cladding 855 thickness of about 20 nm, the width of the mode conversion region may be in the range of about 290-330 nm or about 310 nm. The length of the mode conversion region having a side cladding layer 855 thickness of about 20 nm may be in the range of about 17-32 μm or be about 22 μm, for example.

FIG. 8D shows another layered waveguide system in accordance with various embodiments. Similarly as in FIGS. 8A and 8B, and 8C, FIG. 8D includes a first waveguide layer 820, an optional stop layer 835, and a second waveguide layer 840. FIG. 8D has a similar cladding configuration as that of FIG. 8C except an additional cap cladding layer 870 is formed between the thin side cladding layer 865 and the top cladding layer 864. FIG. 8D also shows a top cladding layer 864 and a bottom cladding layer 866. The cap cladding layer 870 may have various thicknesses (Tc). For example, the cap cladding layer 870 may have a thickness in the range of about 130-210 nm. In some cases, the thickness of the cap cladding layer 870 is 200 nm. In this example, the length of the mode conversion region of the waveguide may be in a range of about 20-40 μm or be in a range of about 20-27 μm. According to various implementations, the length of the mode conversion region is about 22 μm. The width of the mode conversion region of the waveguide may be in a range of about 285-405 nm or may be in a range of about 285-325 nm. According to various implementations, the cap cladding layer 870, top cladding layer 864, and bottom cladding layer 866 comprise SiO2 and the side cladding layer 865 comprises AlO. While each of FIGS. 8A-8D show systems having an optional stop layer, it is to be understood that any of the systems in 8A-8D can be formed without the stop layer as in FIG. 8E, for example.

FIG. 9 illustrates a graph of how the polarization changes as the light propagates through the device in the curved design and the straight-edge design. More specifically, FIG. 9 shows a comparison of the TE purity 930 and the TM purity 940 versus the waveguide position for the curved interface layered waveguide and the TE purity 910 and the TM purity 920 versus the waveguide position for the straight-edge interface layered waveguide. For the curved design, the mode conversion at the beginning and end of the mode conversion region of the waveguide is sped up when compared with the straight-edge design due to a strong etch curvature. The straight-edge design has a more gradual mode conversion at the beginning and end of the mode conversion region of the waveguide. In the mode flip region for the curved design 935, the curvature is more gradual when compared to the mode flip region of the straight-edge design 915.

Table 1 shows the length of the mode conversion region of the waveguide for the straight-edge and the curved design and the resulting transmission efficiencies and TM mode purities.

TABLE 1 Straight-edge Curved Length 27.3 μm 20 μm Transmission 99.5% 99.4% Efficiency TM mode purity 99.9% 99.3%

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.

The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the inventive concepts to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope be limited not with this detailed description, but rather determined by the claims appended hereto. 

What is claimed is:
 1. An apparatus comprising: an input coupler configured to receive light excited by a light source; a near-field transducer (NFT) at a media-facing surface of a write head; and a layered waveguide between the input coupler and the NFT and configured to receive the light output from the input coupler in a transverse electric (TE) mode and deliver the light to the NFT in a transverse magnetic (TM) mode, the layered waveguide comprising: a first layer extending along a light-propagation direction and configured to receive light from the input coupler, the first layer tapering from a first cross track width to a second cross track width, the second cross track width being narrower than the first cross track width; a second layer disposed on the first layer, the second layer having a cross sectional area in a plane perpendicular to the light propagation direction that increases along the light propagation direction, the cross sectional area being smaller proximate to the input coupler and larger proximate to the NFT; and an interface between the first layer and the second layer, the interface comprising a curve.
 2. The apparatus of claim 1, wherein the curve is based on a cubic Bezier curve.
 3. The apparatus of claim 1, wherein the first layer tapers from the second cross-sectional width to a third cross-sectional width as it nears the NFT, the third cross sectional width being wider than the second cross sectional width.
 4. The apparatus of claim 1, further comprising a cladding layer that encompasses sides of the layered waveguide, the cladding layer having a different index of refraction from the first layer and the second layer.
 5. The apparatus of claim 1, further comprising a top cladding layer on a first substrate-parallel surface of the layered waveguide; a side cladding layer that encompasses sides of the layered waveguide and a bottom cladding layer on the side cladding layer, the side cladding layer having a different index of refraction than the top and bottom cladding layers.
 6. The apparatus of claim 1, further comprising a side cladding layer that encompasses sides of the layered waveguide and a first substrate parallel surface of the layered waveguide, a top cladding layer adjacent to the side cladding layer, and a bottom cladding layer adjacent to a side of the first waveguide layer, the side cladding layer having a different index of refraction than the top and bottom cladding layers.
 7. The apparatus of claim 6, wherein the top cladding layer and the bottom cladding layer comprise SiO2 and the side cladding layer comprises AlO.
 8. The apparatus of claim 7, further comprising an intermediate cladding layer between a side parallel to the substrate and the side cladding layer, the intermediate cladding layer having a different index of refraction than the side cladding layer.
 9. The apparatus of claim 8, wherein the intermediate cladding layer comprises AlO.
 10. The apparatus of claim 1, wherein the second cross track width remains substantially constant for a length of about 20 μm.
 11. An apparatus comprising: an input coupler configured to receive light excited by a light source; a near-field transducer (NFT) at a media-facing surface of a write head; and a layered waveguide between the input coupler and the NFT and configured to receive the light output from the input coupler in a transverse electric (TE) mode and deliver the light to the NFT in a transverse magnetic (TM) mode, the layered waveguide comprising: a first layer extending along a light-propagation direction and configured to receive light from the input coupler, the first layer tapering from a first cross track width to a second cross track width, the second cross track width being narrower than the first cross track width; a second layer disposed on the first layer, the second layer having a cross sectional area in a plane perpendicular to the light propagation direction that increases along the light propagation direction, the cross sectional area being smaller proximate to the input coupler and larger proximate to the NFT; and an interface between the first layer and the second layer, the interface comprising a cubic plane curve.
 12. The apparatus of claim 11, wherein the cubic plane curve is based on a Bezier curve.
 13. The apparatus of claim 11, wherein the first layer tapers from the second cross-sectional width to a third cross-sectional width as it nears the NFT, the third cross sectional width being wider than the second cross sectional width.
 14. The apparatus of claim 11, further comprising a cladding layer that encompasses sides of the layered waveguide, the cladding layer having a different index of refraction from the first layer and the second layer.
 15. The apparatus of claim 11, further comprising a top cladding layer on a first substrate-parallel surface of the layered waveguide; a side cladding layer that encompasses sides of the layered waveguide and a bottom cladding layer on the side cladding layer, the side cladding layer having a different index of refraction than the top and bottom cladding layers.
 16. The apparatus of claim 11, further comprising a side cladding layer that encompasses sides of the layered waveguide and a first substrate parallel surface of the layered waveguide, a top cladding layer adjacent to the side cladding layer, and a bottom cladding layer adjacent to a side of the first waveguide layer, the side cladding layer having a different index of refraction than the top and bottom cladding layers.
 17. The apparatus of claim 16, wherein the top cladding layer and the bottom cladding layer comprise SiO2 and the side cladding layer comprises AlO.
 18. The apparatus of claim 17, further comprising an intermediate cladding layer between a side parallel to the substrate and the side cladding layer, the intermediate cladding layer having a different index of refraction than the side cladding layer.
 19. The apparatus of claim 18, wherein the intermediate cladding layer comprises AlO.
 20. The apparatus of claim 11, wherein the second cross track width remains substantially constant for a length of about 20 μm. 